Means for the suppression of parasitic oscillations in a tunable cavity resonator



B. c. GARDNER 2,750,568 HE SUPPRESSION 0F PARA c oscILLATIoNs June l2, 1956 MEANS FOR T SITI IN A TUNABLE CAVITY RESONATOR Filed July 3l 1951 2 Sheets-Sheet l /M/ff/To@ 19t' MVA/P0 C. @M0/VD? y WW Hrm/wey .lune 12, 1956 B. c. GARDNER 2,750,568

MEANS FOR THE SUPPRESSION OF PARASITIC OSCILLATIONS IN A TUNABLE CAVITY RESONATOR Filed July 31, 1951 2 Sheets-Sheet 2 FVG.

4200 4400 4600 4800 5000 5200 $400 :60a Jaaa 6000 6200 HTTOP/Vfy United States Patent C) MEANS FOR THE SUPPRESSION OF PARASITIC gSCT%LATIONS IN A TUNABLE CAVITY RESO- Bernard C. Gardner, Los Altos, Calif., assignor to Raytheon Manufacturing Company, Newton, Mass., a corporation of Delaware Application July 31, 1951, Serial No. 239,425

2 Claims. (Cl. 333--82) This application relates to improvements in cavity resonators and more particularly to the provision of means in such cavities for suppressing undesired frequencies of operation.

Certain types of reilex klystrons adapted for use over a broad band of frequencies, such as type RK-5721, when used in cavity resonators of the type disclosed in the application for United States patent of Philip M. Johnson, Serial No. 132,681, tiled December 13, 1949, now Patent No. 2,712,071, dated June 28, 1955, have been found to oscillate at a set of high frequency repeller modes when vigorously oscillating at the frequency for which they are designed and adjusted. These modes appear to lie between the ordinary modes determined by the repeller voltage and the dimensional adjustments of the cavity. For this reason, and because they occur only when the oscillator is already at nearly its full strength, it is virtually impossible to avoid interference with one of them, as these modes appear midway between the repeller voltages that result in operation at one of the modes represented by a pair of integers, such as (3, 3) and (3, 4). They are designated as half modes (3, 3V2). A theoretical explanation of the occurrence of such modes is set forth by W. H. Huggins in an article entitled, Multifrequency bunching in reflex klystrons, in volume 36 of the Proceedings of the Institute of Radio Engineers, at page 624 (May 1948).

For example, when an oscillator of the type described above is operating in its steady state, simultaneous oscillations can occur on modes (2, 2) (3, 31/2). Since mode (3, 31/2) can only occur when a vigorous oscillation on the desired mode is present, it may be regarded as a parasitic repeller mode. Its presence causes a decrease in power output of the desired mode and it is conceivable that it may produce other complications, such as noise and irregularities in the electronic tuning characteristics.

Unlike interference with the ordinary modes that can occur during pulse build-up but not in the steady state, interference with a parasitic mode cannot occur prior to the appearance of the desired oscillation. It can only develop after the desired oscillation has nearly reached its full amplitude.

Also, the parasitic repeller half modes are only forty per cent. as strong as the ordinary repeller modes and, therefore, may be suppressed readily by selective loading.

By the present invention the tive quarter wave length cavity oscillations in the half modes are suppressed by selective loading in the form of either a tunable or a fixed lossy trap. The tunable trap is in the form of a probe that may be inserted the desired distance into the cavity under control of the tuning dial by means, for example, of a cam and lever arm. The fixed trap is in the form of a lossy resonator having an attenuation band covering the desired frequencies and a pass band covering the undesired frequencies and so positioned in the wall of the main cavity that the motion of the tuning plunger operates as a radio frequency switch.

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Other and further advantages of this invention will be apparent as the description thereof progresses, reference being had to the accompanying drawings, where- Fig. l is a plan view of a cavity resonator for use with a reflex klystron and incorporating a preferred form of the invention;

Fig. 2 is an end View of the cavity of Fig. l;

Fig. 3 is a section along the line 3-3 of Fig. 2 broken away to `show only the left end of the cavity as shown in Fig. 1;

Fig. 4 is an enlarged sectional view of the trap shown in Fig. 3;

Fig. 5 is a graph of the varying depths of penetration of the probe of the trap of Fig. 4 for different frequencies.

In Figs. 1, 2 and 3, there is shown a cavity resonator having a supporting base 11. At one end of the base 11, there is attached a support member 12. The axis of the hole in the member 12 extends parallel to the base member 11. The support member 12 supports a cylinder 13 coaxial therewith which forms the outer member of a coaxial cavity resonator of the type fully described in the above-cited application of Philip M. Johnson. One end of the cylinder 13 extends outwardly beyond the end of the base member 11 and terminates in a cap assembly 14 designed to engage the lower grid 15 of the velocity modulation chamber of a klystron 16. The cap assembly 14 comprises an annular disk 17 attached to the end of cylinder 13 by screws 18, said disk 17 having a hole therein of suicient diameter to allow the entrance of the external portion of the lower grid 15 of the klystron 16. Resting against the outer side of the disk17, and snugly engaging grid 15, is a resilient wire coil of a spring-like material which is urged against the disk 17 by an annular wedge-shaped member 21 and a cap member 22 threaded on to the disk 17. The compression of wire spring 20 by the wedge-shaped member 21 causes circumferential expansion thereof both outwardly and inwardly, thereby firmly grasping the grid 15.

The klystron 16 is of the well-known reex type having a single modulation chamber comprising upper and lower modulation grids 23 and 15. The details of the klystron 16 will not be further described as they have been fully described elsewhere (see the cited copending application of Philip M. Johnson) and they form no part of the present invention.

An output coupling 24 is inserted into one side of the cylinder 13. This coupling 24 comprises an outer shield member threaded into the wall of the cylinder 13 and a central conductor coaxial with the shield member whose inner end is formed into a pick-up loop and attached to the inner end of the shield member of the coupling 24.

The upper grid 23 is extended to be contacted by lingers 25 formed on the central conductor 26 of the coaxial cavity resonator 10.

Surrounding cylinder 13 is an outer cylinder 27 which rests in a second support member 28 attached to the base 11. The cylinder 27 carries thereon a cam follower support member 30 rigidly attached thereto by a bolt 31. This operates in a manner fully described in the cited Johnson application to cause the rotation of cylinder 27 to be communicated through a cam to move the shorting device toward and away from the klystron 16.

The right-hand end of the cylinder 27 terminates in an end plate 32 rigidly attached to the cylinder 27. The end plate 32 is formed with a hole therein coaxial with the cylinder 27 into which is inserted a cylindrical insulating member 33 which is rigidly held with respect to end plate 32. The insulating member 33 extends into a potentiometer 34, which may be of any desired type which will track with the particular characteristics of the tube used. This potentiometer 34 controls the voltage applied to the reflector electrode of the klystron. The details of a potentiometer suitable for this purpose have been fully described in the cited application. The insulating member 33 extends through the potentiometer 34 and is rigidly attached to a knob 35 and indicating dial 36. The result is that as the knob 35 is turned the potentiometer 34 selects a voltage to be applied to the reflector electrode of the klystron 16 that is appropriate for the resonant frequency of the cavity determined by the position of the shorting bar as set by the rotation of the cylinder 27, also controlled by the position of the knob 35.

Opposite the output coupling 24 and slightly to its right, as seen in Fig. 1, a tuned trap 37 is inserted in the cylinder 13. The details of this trap are shown in Fig. 4. The trap 37 is formed of a cylindrical case 38 terminated in a threaded neck 40 that screws into the wall 13 of the resonator 10. Axially mounted within this case 38 is a rod 41 that penetrates into the cavity 10 within the space between the outer conductor 13 and the inner conductor 26. The rod 41 is held in this position by a piston 42 slidably mounted within the case 38. The rod 41 passes through a hole in the piston 42 and is kept from axial movement with respect to the piston 42 by a set screw 43. Radio frequency energy is prevented from penetrating into the space within the case 38 by a noncontacting choke joint 44. This joint effectively presents a short circuit to the radio frequency currents in the outer conductor 13 while preventing any actual contact between the rod 41 and the outer conductor 13 at this point. If it is desired to maintain the cavity at a particular atmospheric pressure, a washer 45 of insulating material can be inserted across the opening into the cavity 10.

With this construction only that portion of the rod 41 which is within the cavity 10 at any time affects the radio frequency characteristics of the oscillator. The portion of the rod 41 which penetrates into the cavity proper affords a resonant circuit to a particular frequency of radio energy dependent upon the length of this portion of the rod. The manner in which this frequency varies with the length of penetration of the rod 41 can be seen in the graph 46 in Fig. 5. This frequency will be lower as the probe penetrates further into the cavity, and higher as it is retracted until ultimately the probe is withdrawn to such an extent that it no longer couples to the fields in the main resonator. The graph shown is that for a trap designed for the frequency range 4,290 to 6,200 megacycles for use in a cavity and oscillator designed to operate in the range from 4,290 to 8,340 megacycles in the (2, 2) mode with suppression of the ve quarter Wave length (3, 31/2) interfering half mode.

It was found that the frequency of the interfering mode is the same as that covered at the high frequency end of the desired tuning range of the resonator. The trap designed for this interfering mode must, therefore, be decoupled when the oscillator is operated at the high frequency end of its tuning range.

By this invention the extent of penetration of the rod 41 into the cavity 10 is controlled by a cam 47 shown in Figs. 1, 2 and 3 attached to the rotating sleeve 27 that is controlled by the tuning knob 35. The rod 41 is connected to a lever 48 which is pivoted at a point 50 to a post 51. The right-hand end 52 of the lever 48 is pressed into contact with the surface 53 of the cam 47 by a spring 54 aixed to the post 51 and arranged to bear against the end 52 of the lever 48 to maintain it in contact with the surface 53 of the cam 47 at all times. As the carn 47 is rotated with the cylinder 27 by the knob 35, it presents a varyingradius to the lever end 52, forcing it out 0r in, according to its position. This motion is communicated by the lever 48 to the rod 41 to vary its penetration into the cavity 10, and thus to vary the frequency which it absorbs from the cavity to be that of the undesired half mode.

With a cavity designed for the three-quarter wave length (2, 2) mode for the frequencies in the band from 4,290 to 8,340, it is desirable to design the cam lever system and trap so that the penetration of the rod 41 varies between zero and .16 inch. The penetration as shown in Fig. 5 varies from almost the maximum when the cavity is tuned to 4,200 megacycles to about .08 of an inch when the cavity is tuned to 4,650 megacycles. it is then extended further until it is fully extended when the cavity is tuned to a frequency of 5,250 megacycles and again retracted until it is fully withdrawn when the cavity is tuned to a frequency of 6,200 megacycles. lt remains fully withdrawn until it reaches the range of the cavity, namely, to 8,340 megacycles.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. In a coaxial cavity resonator for use with a velocity modulation electron discharge device of the reex type, means for adjusting said coaxial cavity to resonate at a plurality of frequencies, means for suppressing undesired modes of oscillation comprising a probe resonant at one of the frequencies of the undesired modes of oscillation projecting radially into the cavity resonator for dissipating the energy at the undesired frequencies as determined by the depth of penetration of the probe, and means under control of said frequency adjusting means to vary the coupling of said suppressing means to said resonant cavity, to maintain the effectiveness of said suppressing means over a certain range of frequencies and ineffective at another range of frequencies of oscillation by controlling the depth of penetration of said probe.

2. In a coaxial cavity resonator for use with a velocity modulation electron discharge device of the reflex type, means for adjusting said coaxial cavity to resonate at a plurality of frequencies, means for suppressing undesired modes of oscillation comprising a probe resonant at one of the frequencies of the undesired modes of oscillation projecting radially into the cavity resonator for dissipating the energy at the undesired frequencies as determined by the depth of penetration of the probe, and cam and lever means under control of said frequency adjusting means to vary the penetration of said probe into said resonant cavity to maintain the effectiveness of said suppressing means over a certain range of frequencies and ineifective at another range of frequencies of oscillation.

References Cited in the file of this patent UNITED STATES PATENTS 2,088,749 King Aug. 3, 1937 2,422,028 Martin June 10, 1947 2,425,345 Ring Aug. 12, 1947 2,481,993 Fuss Sept. 13, 1949 2,485,030 Bradley Oct. 18, 1949 2,593,095 Brehm Apr. 15, 1952 2,652,5.11 Hewlett et al Sept. 15, 1953 2,688,122 Edson et al Aug. 31, 1954 

